1. Field of the Invention
The present invention relates generally to gas bearing turbocompressors. More particularly, the invention concerns turbocompressors embodying a novel, uniquely configured thrust washer in which centrifugal stresses are significantly reduced.
2. Discussion of the Prior Art
Turbocompressors have been in use as turbochargers for internal combustion engines for many years. These types of turbocompressors have generally embodied oil-lubricated bearings. The trend in recent years in turbocompressor design has been towards higher compression ratios requiring higher revolutions per minute (RPM) of the spindle of the turbocompressor and resulting in higher exhaust temperatures. Under such conditions, oil lubrication of the bearings becomes inadequate and can possibly result in cavitation in the bearings as a result of the higher rubbing speed, and in thermal decomposition as a consequence of the higher temperatures. A solution to the aforementioned problems is provided by the use of gas bearings such as the bearings disclosed in U.S. Pat. No. 4,808,070 issued to the present inventor. The novel gas bearings disclosed in U.S. Pat. No. 4,808,070 can easily handle the required RPM and rubbing speeds of most types of modem turbo compressors.
Another application of oil free bearings is in the air feed of fuel cells which are poisoned by oil vapor.
In a typical turbocharger the compressor is fed through a filter that causes a significant pressure drop at the inlet to the compressor resulting in an inlet pressure lower than the atmosphere. On the other hand, the conventional turbine discharges to atmosphere through a muffler or catalytic converter also causing a pressure drop which results in a pressure higher than atmosphere at the exhaust of the turbine. The resulting pressure difference between the compressor housing and the turbine housing causes a net thrust to develop in the shaft connecting the compressor wheel and the turbine wheel. In an oil-lubricated turbocharger this thrust is absorbed by a traditional oil-lubricated thrust washer. However, in the case of gas bearings the thrust is typically compensated by the arrangement described in U.S. Pat. No. 5,567,129 issued to the present inventor. Because of the relevance of the U.S. Pat. Nos. 4,808,070 and 5,567,129 to a complete understanding of the present invention, both of these patents are hereby incorporated by reference as though fully set forth herein.
U.S. Pat. No. 6,231,302 entitled Thermal Control System for Gas Bearing turbocompressor, also issued to the present inventor is directed toward solving the problems caused by large temperature differences between the ends of the turbocompressor. This latter patent, U.S. Pat. No. 6,231,302, is also incorporated by reference as though fully set forth herein.
An important component of the turbocompressor disclosed in U.S. Pat. No. 6,231,302 is the thrust washer (therein described as xe2x80x9cthird wheelxe2x80x9d) which is there provided in the form of a thin disk that is mounted on a rotating shaft. The thrust washer of necessity is provided with a hole in the center that is needed for the physical assembly of the turbocompressor. It is, of course, fundamental that the central hole weakens the disk against stresses generated by centrifugal forces when the disk is spinning at high speed. Such stresses increase with the diameter of the disk as well as the square of the angular speed and may exceed the strength of the material resulting in the destruction of the disk if either diameter or angular speed exceed a limiting value. Since the angular speed is predetermined by gas dynamic requirements of the turbine and compressor wheels, the diameter is limited by the strength of the material. However, the diameter is desired to be as large as possible because the aerodynamic stiffness of the thrust bearing is proportional to the square of the diameter.
One objective of the present invention is the reduction of the centrifugal stresses in the apertured disk so as to allow an increase in the diameter of the disk and consequently an increase in the aerodynamic stiffness of the bearing. Another object of the invention is to accomplish the reduction of the centrifugal stresses while retaining the flatness and parallelism of the faces of the disk so as to maintain the gas dynamic performance of the thrust bearings. Still another object is to reduce the stresses at the center hole of the disk by removing existing stress concentrations resulting from auxiliary openings in the disk such as those found in the prior art.
It is well known that the stressing a spinning wheel or disk is largest at the edges of a central hole, and that this stress can be substantially decreased by tapering the axial thickness of the wheel from a maximum near the center to a minimum at the edge.
It is also well known that holes in a plate under stress double the tangential stress at the edge of the hole and thereby act as stress concentrators. Since the design of the high temperature thrust bearing requires communication of gas from one face to the other, the apertures 179 provided in the disk 162 as shown in FIG. 3 of U.S. Pat. No. 6,231,302 are such stress concentrators. In the manner to be described hereinafter, this stress can be reduced by transferring the openings from the disk to the rotating shaft.
It is an object of the present invention to provide an improved gas bearing system for use in a turbocompressor or similar device, such as that disclosed in incorporated by reference U.S. Pat. No. 6,231,302 which embodies a uniquely configured thrust washer or disk that exhibits superior strength characteristics when the disk is rotated at high speed. More particularly, it is an object of the invention to provide such a thrust washer in which the stresses found at the edges of the central hole in the disk are substantially decreased by tapering the axial thickness of the disk from a maximum near the center of the disk to a minimum proximate the edge of the disk.
Another object of the invention is to accomplish a reduction in the centrifugal stresses in the thrust washer so as to allow an increase in the diameter of the disk and consequently an increase in the aerodynamic stiffness of the bearing.
Another object of the invention is to accomplish a reduction in the centrifugal stresses while retaining the flatness and parallelism of the faces of the disk so as to retain the gas dynamic performance of the thrust bearings.
Another object of the invention is to provide a thrust washer as described in the preceding paragraphs which is generally circular shaped disk, has a predetermined diameter and comprises a body portion having spaced apart faces and a groove formed intermediate the spaced apart faces to define a pair of radially outwardly extending segments.
Another object of the invention is to provide a thrust washer as described in the preceding paragraph in which the groove formed intermediate the spaced apart faces has sloping sidewalls and has a depth generally greater than one-half the radius of the thrust washer.
These and other objects are realized by the turbocompressor described in the paragraphs that follow.